Process to recover base metals

ABSTRACT

A hydrometallurgical process to leach at least one base metal from one or more base metal sulphide feeds comprising the steps of providing a base metal sulphide feed; processing at least a portion of the feed by roasting to reduce the content of sulphur therein to a predetermined sulphur content level; directing some or all of the processed feed with or without some of the unprocessed feed to an oxidative leach step to carry out an oxidative leach step under conditions sufficient to recover at least a portion of the base metal from the feed, the predetermined sulphur content level being selected so as not to exceed that which is required to provide a source of heat for the oxidative leach step and to provide sufficient sulphate to complex the portion of the base metal being recovered

REFERENCE TO CO-PENDING APPLICATION

The entire subject matter of U.S. Provisional application Ser. No. 60/545,437 filed Feb. 18, 2004 and entitled PROCESS TO RECOVER BASE METALS is incorporated by reference. The applicant claims priority benefit under Title 35, United States Code, Section 119(e) of U.S. Provisional application Ser. 60/545,437 filed Feb. 18, 2004 and entitled PROCESS TO RECOVER BASE METALS.

FIELD OF THE INVENTION

This invention relates to the hydrometallurgical processing of metals from sulphide feed materials.

DESCRIPTION OF THE RELATED ART

The hydrometallurgical treatment of base metal sulphide feeds such as chalcopyrite (copper) and pentlandite (nickel) concentrates has been the subject of numerous investigations, since it would constitute an alternative to conventional smelting. 20 Several new processes have been developed, particularly for the treatment of copper sulphide concentrates;

to cite a few of them: the INTEC process (atmospheric chloride process), the thermophile bioleach processes (known by the names BIOCOP, MINTEK/BACTECH), low temperature pressure oxidative leaching processes (known by the names ACTIVOX, MIM), medium temperature pressure oxidative leaching process (known by the names CESL, ANGLO AMERICAN, PHELPS DODGE, DYNATEC), and high temperature pressure oxidative leaching process (known by the names PLACER DOME, PLATSOL®).

Low temperature leaching processes (typically operating between 90 and 110° C.), require ultra fine grinding (<10 microns) and long leach residence times (>2 hours) to achieve acceptable copper recovery. Medium temperature processes (typically operating at 130 to 170° C.) have to take special precautions to overcome the potential negative effect of elemental sulphur, due to armouring of the sulphide surfaces by elemental sulphur for example. One process adds chloride ions, another adds a carbon compound, while others add a dispersant, all with the objective of mitigating the negative effect of elemental sulphur.

High temperature pressure oxidation (HTPOX) processes (such as the PLACER DOME process and the PLATSOL® process, which operate at 190 to 230° C.) do not require ultrafine grinding, are extremely fast, and do not form elemental sulphur (forming instead sulphate, mostly in the form of sulphuric acid). The negative aspect of the high temperature process is that the equipment to perform the process is relatively more expensive. In addition oxidation of sulphide ions all the way to sulphuric acid consumes much more oxygen in the autoclave, and in those cases where there is no application for the acid that is produced, it has to be neutralized at the expense of a neutralizing agent.

It is an object of the invention to benefit from at least some of the advantages of the low, medium and/or high temperature leaching processes, while reducing at least some of their disadvantages.

SUMMARY OF THE INVENTION

In one of its aspects the present invention provides a hydrometallurgical process to recover at least one base metal from one or more base metal sulphide feeds comprising the steps of:

-   -   providing a base metal sulphide feed;     -   processing at least a portion the feed to reduce the content of         sulphur therein to a predetermined sulphur content level;     -   directing some or all of the processed feed with or without some         or all of the unprocessed feed to an oxidative leach step to         carry out an oxidative leach step under conditions sufficient to         recover at least a portion of the base metal from the feed, the         predetermined sulphur content level being selected to provide a         source of heat for the oxidative leach step and to provide         sufficient sulphate ions to complex the portion of the base         metal being recovered.

Preferably, the predetermined sulphur content level does not exceed that which is required to provide a source of heat for the oxidative leach step and to provide sufficient sulphate to complex the portion of the base metal being recovered.

The oxidative leach step may be carried out either under ambient pressure conditions or above-ambient pressure conditions, depending on a number of different factors, such as the presence of PGM's, (in which case the oxidative leach step may likely be carried out at above-ambient pressure conditions), as well as other economic and environmental factors. The oxidative leach step may be carried out in an autoclave, although it may also be carried out in another reactor such as an open tank or atmospheric vessel.

In one embodiment, the directing step is carried out to recover substantially all of the recoverable base metal from the feed. However, the process may also be conducted to recover a predetermined portion of the base metal contained in the feed.

The entire feed or a portion of the feed may be subjected to the processing step which may involve roasting under either auto thermal conditions or externally heated conditions. The roasting step itself may be a partial roast (meaning that sulphides remain in the roast product) or a substantially complete or “dead” roast (meaning that substantially all sulphides have been removed from the roast product).

In one embodiment, the roasting step occurs under auto thermal conditions or under conditions where an external heat source delivers heat thereto. The roasting step may occur in a roasting vessel or chamber.

In one embodiment, the processing step may include the step of dividing the base metal sulphide feed into a first portion and a second portion, the first portion being subjected to the roasting step to form a roasted first portion. In this latter case, the second portion may be directed to an oxidative leach step together with at least a portion of the roasted first portion.

This may be carried out in one, or more than one, reaction vessels either under atmospheric or non-atmospheric conditions. The reaction vessels may themselves include a number of compartments in series along one or more flow paths which themselves may be parallel. Alternatively or in addition, the flow path may include a number of vessels ganged in series. The reaction vessels may include one or more autoclaves or agitation tanks and the like.

Thus, in one embodiment, the step of directing the feed includes injecting the roasted first portion at different locations in the one or more reaction vessels. In one example, the one or more reaction vessels includes one or more autoclaves and/or one or more agitation tanks. If desired, one or more of the reaction vessels may form one or more flow paths, which themselves may be in series or parallel relationship. If desired, one or more of the reaction vessels may include a number of compartments in series along one or more of the flow paths

In one embodiment, the process further comprises the step of subjecting the first portion to auto thermal roasting conditions in the presence of air or oxygen to produce the calcine and sulphur dioxide (SO₂). In this case, the SO₂ is removed from the roaster and delivered to an Acid Plant to be converted to sulphuric acid (H₂SO₄) or liquid SO_(2.)

In one embodiment, the process further comprises the step of recovering sulphur dioxide from the roasting step and converting the sulphur dioxide to sulphuric acid.

In one embodiment, the roasted first portion is substantially entirely depleted of sulphide. In this case, the roasted first portion may be totally or partially blended with the second portion prior to delivery to the autoclave.

In one embodiment, the step of directing the feed includes injecting the roasted first portion at different locations in the autoclave or other reaction vessel as the case may be. Alternatively, the roasted first portion may be delivered to just one location in the autoclave, such as an upstream end thereof, or at a location upstream of the autoclave itself.

In one embodiment, the process further comprises the steps of:

-   -   collecting a pressure leach slurry from the autoclave; and         thereafter     -   directing an additional portion of the roasted first portion to         the pressure leach slurry.

Preferably, the oxidative leach step occurs at temperatures ranging from about 40 to about 250 degrees Celsius except in circumstances where there is platinum or other precious metals in the base metal sulphide feed in which case a pressure vessel is required, such as an autoclave, and preferred temperatures may range from about of 190 to about 230 degrees Celsius when using the PLATSOL process to recover the precious metal. The gold, or platinum group metals may be recovered using either the PLATSOL step or a subsequent cyanidation step, or another appropriate step.

Other temperature ranges may also be employed, provided the base metals are dissolved and/or the formation of elemental sulphur is minimized. For example, in the case of secondary copper minerals such as chalcocite and covellite, the temperature of the oxidative leach step may be lower, since such materials are easier to oxidize.

Preferably, the roasted first portion ranges from about 5 percent to about 100 percent of the base metal sulphide feed material. Either portion 100% roasted or partially, the portion that is roasted will be partially roasted under mild conditions to benefit from more favourable kinetics and thermodynamics that prevail in the initial stages of roasting. It is expected that 40 to 100 percent of the sulphur will be oxidized to SO₂ in this step. The specific conditions for a particular sulphide feed will depend on the feed itself which may include chalcopyrite, nickel copper sulphide, nickel copper sulphide containing PGMs and /or gold.

Preferably, the metal sulphide feed material is a concentrate formed from a flotation step, although other processes may be employed, such as gravity separation and magnetic separation. For example, the first portion may be a relatively high grade cleaner concentrate (having a sulphur content, for example, ranging from 20 to 40 percent), while the second portion may be a relatively low grade cleaner tailings scavenger concentrate (having a sulphur concentrate, for example, of less than 20 percent).

In one embodiment, the oxidative leach step generates a slurry temperature within the autoclave ranging from about 190 to about 250 degrees Celsius.

In one embodiment, the pulp density in the feed ranges from about 5 to about 30 percent.

In one embodiment, the first portion ranges from about 5 percent to about 100 percent of the feed.

In one embodiment, the one or more base metal sulphide feeds includes a concentrate formed from a flotation step, a gravity concentration step or a gravity/flotation concentration step. In one example, the first and second portions may originate from different feeds.

Preferably, the oxidative leach step generates a terminal sulphuric acid concentration not exceeding 80 g/L, and more preferably not exceeding 50 g/L, in order to capture iron constituents in the feed material as stable iron solid residues.

In one embodiment, the process includes the step of collecting a pressure leach slurry from the autoclave. In this case, the pressure leach slurry may contain residual free acid, in which case a sufficient portion of the roast product can be added to the pressure leach slurry to neutralize the residual free acid.

In another embodiment, the oxidative leach step is carried out in a heap. In this case, the material in the heap should be prepared or arranged to allow effective distribution of the leach solution therethrough. For instance, the heap may be prepared by the use of granular carrier elements on which the processed feed (or the mixture with unprocessed feed as the case may be) has been deposited. Other methods may also be used or become available for agglomerating or establishing a porous matrix of the processed feed in order to receive the leach solution.

In one embodiment, the oxidative leach step occurs in the presence of sulphuric acid at a concentration not exceeding 80 g/L, in order to facilitate the capture of unwanted iron impurities in a stable hematite residue.

In another of its aspects, the present invention provides a hydrometallurgical process to recover at least one base metal from a base metal bearing sulphide feed material comprising the steps of:

-   -   providing a base metal bearing sulphide feed material;     -   splitting the base metal bearing sulphide feed material into a         first portion and second portion;     -   removing substantially all the sulphide from the first portion         to form a sulphide depleted first portion;     -   collecting the sulphide depleted first portion;     -   directing at least a portion of the sulphide depleted portion to         an autoclave;     -   directing the second portion to the same autoclave to carry out         an oxidative pressure leach step to recover the base metal from         the feed, the second portion having sufficient sulphide content         to act as the source of heat for the oxidative pressure leach         step.

For those base metal sulphides containing gold or other “platinum group metals” (PGM's), the latter may be solubilized by adding a halide, more preferably a reactive chloride as described below.

In still another of its aspects, the present invention provides a process to recover a base metal, comprising the steps of:

-   -   providing a feed material containing a base metal sulphide         constituent with a predetermined amount of a sulphur-bearing         constituent therein;     -   directing the feed material to a reactor to carry out an         oxidative leach step under conditions sufficient to recover an         economic portion of the base metal from the feed material     -   wherein the providing step includes adjusting the predetermined         amount of the sulphur-bearing constituent, so as not         substantially to exceed the amount required to provide a source         of heat for the oxidative leach step and to provide sufficient         sulphate to complex the portion of the base metal being         recovered.

In an embodiment, substantially all of the predetermined amount of sulphur-bearing constituent in the feed material upstream of the leach reactor is in a form other than a sulphate. Preferably, substantially all of the predetermined amount of sulphur-bearing constituent in the feed material in the reactor is converted to a sulphate for complexing with the base metal.

In an embodiment, the providing step includes the step of roasting at least a portion of the feed material. If desired, the feed material may be divided into a first portion and a second portion, followed by roasting the first portion.

If desired, the step of providing may include the step of providing a first portion of the feed material with substantially no sulphur-bearing constituent and a second portion containing a known amount of the sulphur-bearing constituent, and mixing the first and second portions.

In an embodiment, the roast product is substantially void of ferrite-bearing materials.

BRIEF DESCRIPTION OF THE DRAWINGS

Several preferred embodiments of the present invention will be provided, by way of examples only, with reference to the appended drawings, wherein:

FIG. 1 is a flow chart illustrating a hydrometallurgical process to leach at least one base metal; and

FIG. 2 is a flow chat illustrating one example of the process of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, there is provided a hydrometallurgical process to leach at least one base metal from a base metal sulphide feed material 4 by utilizing the advantages of the high temperature HTPOX processes.

The process involves dividing the feed material into a first portion X shown at 8 and a second portion Y shown at 10. It will be understood that, as an alternative, the first and second portions may originate from separate feeds, as represented by the dashed lines 8′, 10′.

The first portion is then directed to a roasting station 12 where the first portion 8 is roasted, in the presence of air or oxygen-enriched air or oxygen, to form a roast product in the form of a calcine 16, in which the sulphur (that is elemental sulphur and other sulphur bearing constituents such as sulphide materials) is either partially or essentially completely depleted.

At least a fraction of the calcine 16 is then delivered to a feed to a closed vessel in the form of an autoclave 18, together with the unroasted second portion 10, where the calcine 16 and second portion 10 are subjected to an oxidative pressure leach step in the autoclave 18 by oxygen or air to recover at least some of the base metal contained in the first or second portions.

The SO₂ gas in the roasting step is converted to H₂SO₄ in an acid plant 14, for example at a concentration of 93 percent or, optionally, about 98 percent for commercial grade H₂SO₄. The acid so produced can be reused in the process, sold, or stored.

As will be described, a particular feature of the present method is that the amount of the sulphur (in the form of elemental sulphur and sulphide) present in the combined feed to the leaching process is sufficient to provide auto thermal conditions for the oxidative pressure leach step to occur at the required temperature, while the first portion 8 is oxidized using air or oxygen, or oxygen-enriched air in an inexpensive roasting step to reduce, if not substantially eliminate, oxygen-consuming sulphur constituents therein.

In this example, the base metal sulphide feed material 4 may be a concentrate of chalcopyrite (CuFeS₂) or of pentlandite (Fe,Ni)₉S₈ or other materials such as pyrite, arsenopyrite, bornite, enargite, chalcocite, sphalerite, pyrrhotite, covellite, cobaltite, millerite or combinations of one or more thereof. The concentrate may be produced by a number of well known processes such as flotation or gravity processes.

When considering chalcopyrite concentrates (CuFeS₂), it is apparent that there are 2 moles of sulphur per mole of copper, and that in the HTPOX process (PLACER DOME, PLATSOL®), all the sulphur eventually has to be neutralized to form gypsum, in addition to all the sulphur originating from other sulphides in the concentrate such as pyrite, pyrrhotite and others.

The proportions of the base metal sulphide feed material fractions X and Y can depend on many factors, such as total sulphide in the concentrate and the acid-consuming properties of gangue minerals in the concentrate. Preferably, the amount of un-roasted portion proceeding directly to the autoclave 18 must be sufficient to generate auto thermal conditions for the combined feed. In other words, the unroasted portion provides sufficient oxygen-consuming sulphur constituents so that their oxidation generates sufficient heat in the autoclave 18 to drive the reactions occurring within the autoclave 18 substantially without another (such as external) source of heat. However, there may be some cases where supplemental heat is necessary or desirable. For example, there may be cases where the temperature of the autoclave 18 may be monitored and, should the sulphur content of the feed momentarily drop below a minimum value, a supplemental heat source may be introduced to make up the difference, that is to compensate for the loss of heat.

It is desirable that there be sufficient sulphate formed in the autoclave 18 to react with the base metals to form base metal sulphates which will thus become soluble and leave the autoclave 18 in a leach solution. In this case, a significant portion of the sulphates is likely to be formed in the autoclave 18, though the roast product may also be a source of sulphates in cases where the roasting step is carried out in conditions forming them. Preferably, the roasting step may be configured to effectively eliminate sulphate since the sulphates otherwise present in the roast product will increase the amount of lime needed for acid neutralization, adding cost to the process. Sulphate formation may be minimized in the roasting step by selecting appropriate temperature and partial oxygen pressure conditions. It is, thus, desirable that there be sufficient sulphides in the feed entering the autoclave 18, not only to provide the source of heat to drive the reactions occurring therein, but also to provide sufficient sulphates for the complexing of base metals of interest (Cu, Ni, Co, Zn).

In one example, the size of the first portion 8 may vary between about 5% and about 100% of the total base metal sulphide concentrate feed 4.

The calcine 16 may be delivered to the autoclave 18 after being totally or partially blended with the second portion 10. Alternatively a portion or substantially all of the calcine 16 may be injected or otherwise deposited to the autoclave 18 at different locations therein.

In some cases, it may be desirable to direct still another portion of the calcine 16 to the pressure leach slurry produced after being recovered from the autoclave 18. This can be useful in some cases to neutralize excess acid in the leach slurry without having to use lime.

The autoclave 18 may be conditioned to carry out a high temperature POX process which can be a simple high temperature oxidation process. Alternatively, a reactive halide (such as chloride) constituent may be added if the sulphide concentrate and calcine contain precious metals such as gold and the platinum group metals (PGM's) by using the well known PLATSOL® conditions, for example using the conditions of U.S. Pat. No. 6,315,812 entitled OXIDATIVE PRESSURE LEACH RECOVERY USING HALIDE IONS, which issued Nov. 13, 2001, the entire subject matter of which is incorporated herein by reference. In this case, the gold and PGM's may be solubilized in a halide complex in the leach solution.

In some applications, it may be advantageous to produce two concentrates in the initial flotation or gravity/flotation process, a high grade cleaner concentrate and a lower grade cleaner tailings scavenger concentrate and the overall recovery of valuable metals in the flotation or gravity/flotation process may be improved by adopting this approach. The cleaner concentrate may contain a high concentration of sulphides and base metals, and thus may constitute the first portion because it can be readily treated in a roaster, producing a concentrated stream of sulphur dioxide gas for conversion to sulphuric acid. The lower grade cleaner tailings concentrate would contain some gangue minerals, which have minimal negative impact on the operation of the autoclave, but which are recovered to enhance the overall recovery of valuable metals (particularly gold and PGMs) in the overall process.

The pulp density (that is the concentration of solids per unit volume) in the feed to the autoclave 18 in HTPOX processes (PLACER DOME and PLATSOL®) is dependent on the concentration of the sulphide in the concentrate and the operating temperature in the autoclave 18. For example the pulp density is usually quite low and in the 5 to 30% range for two reasons:

-   -   (i) Oxidation of sulphides is an exothermal process. The above         pulp densities (5 to 30%) produce auto thermal conditions with         high sulphide grade concentrate, where the heat generated by the         oxidative pressure leach step generates the required operating         temperature in the autoclave 18 (190 to 230° C. for HTPOX). At         higher pulp densities, heat may need to be extracted from the         autoclave 18, which in some cases increases operating costs; at         lower pulp densities, the feed to the autoclave 18 may need to         be preheated to maintain the required temperatures in the         autoclave 18, which adds to the capital cost of the plant.         Operating at the optimum pulp density for a given sulphur         content of the feed produces the most favourable economics for         the process.     -   (ii) A typical chalcopyrite concentrate will have a sulphide         concentration of about 30%, which will potentially generate up         to 1 ton of sulphuric acid per ton of concentrate, when fully         oxidized. To produce a stable iron residue of hematite during         pressure leaching (which is desirable for environmental as well         as processing reasons), the sulphuric acid concentration in the         autoclave must be maintained at <80 g/L, and preferably <50 g/L.         This means that the pulp density in the feed to the autoclave 18         should be adjusted to take into account the acid consuming         gangue mineral constituents in the concentrate. For example, the         required pulp density to produce hematite in the residue may be         less than 10% in some cases.

Not withstanding the economic benefits of operating the oxidative leach step under auto thermal conditions in the autoclave and the benefits of producing a stable iron residue such as hematite, it may, in some cases, be desirable to feed the autoclave at a much higher pulp density, to reduce the size of the reaction vessel and lower the capital cost of the reaction vessel, and to minimize the amount of water that wastefully consumes heat in the process.

The calcine 16 that is added to the feed to the autoclave 18 in the present process is an acid-consumer, so by blending the first portion 8 into the autoclave feed with the unroasted second portion 10, it will be possible to feed the autoclave 18 at much higher pulp densities, while still meeting the requirements for auto thermal operation and residual sulphuric acid of <80 g/L. This will positively impact the capital cost of the overall process by significantly reducing the size of the capital-intensive autoclave 18. For example, doubling the overall pulp density to about 20% solids will more than halve the size of the autoclave 18 compared to stand alone HTPOX or PLATSOL® processes.

The calcine 16 can be totally or partially blended with the unroasted concentrate of the second portion 10 prior to entering the autoclave 18, or it could be injected at different locations inside the autoclave 18, or part of the calcine 16 may be added to the pressure leached slurry after discharging from the autoclave 18, to neutralize the residual free acid under properly selected conditions. The process will generate a hematite iron residue, which is easily discarded without substantial short or long term impact to the environment.

If the concentrate contains gold or PGM minerals, the precious metals can be recovered from the combined calcine 16/HTPOX leach residue by conventional means such as the cyanidation process, since the residue does not contain elemental sulphur. Gold and the PGMs can also be recovered directly into the acidic autoclave liquor by using PLATSOL® conditions.

The advantage of PLATSOL® over conventional cyanidation for the treatment of the oxidized concentrate are:

-   -   the capital and operating expenses of the cyanidation plant are         avoided     -   the high consumption of cyanide associated with treatment of         base metal leach residues can be substantially avoided;     -   the PLATSOL® process efficiently recovers gold as well as PGMs,         whereas cyanidation only recovers gold efficiently;     -   the PLATSOL® process is a viable alternative to cyanidation for         gold recovery in those regions of the world where permitting of         the cyanidation process is restricted and;     -   gold can be recovered directly from the oxidized, acidic slurry         by carbon in pulp, prior to solid liquid separation, reducing         the risk of gold losses during solid liquid separation;

Thus, examples of the present process may have one or more of the following advantages:

-   -   relatively fast kinetics for base metal (such as Cu, Ni)         leaching and iron precipitation as hematite;     -   minimal environmental impact from the hematitic tailings;     -   generally no requirement for ultrafine grinding;     -   no requirement for the addition of chloride (unless PLATSOL®),         nor carbon, nor dispersant, to avoid passivation of the sulphide         mineral surfaces;     -   excellent base metal (Cu, Ni, others) recoveries (>98%);     -   low oxygen consumption;     -   low neutralizing agent consumption;     -   no new equipment required, but a combination of a well known         roasting and HTPOX autoclave equipment, operating under proven,         well-established conditions, similar to refractory gold plants         for the autoclave and copper nickel roasting for the roaster.     -   relatively easy gold recovery from the POX residue, with low         consumption of cyanide, since it does not contain elemental         sulphur, or during HTPOX if applying the PLATSOL® process.     -   direct recovery of PGMs via the PLATSOL® process at minimal         incremental cost versus base metal recovery costs;     -   the potential for increasing overall recovery of pay metals to         the concentrate by producing a high grade (cleaner) concentrate         for processing in the roaster and a lower grade (cleaner         tailings) scavenger concentrate for treatment in the autoclave;     -   low operating costs because a significant amount of sulphide is         oxidized in the roasting step with the inexpensive oxidant air,         (versus oxidizing all the sulphide with expensive oxygen in the         HTPOX process), and because concentrated sulphuric acid is         produced from the roaster, which can either be used in the         process or sold to generate additional revenue, versus the         requirement to neutralize autoclave acid with limestone or other         agents; and     -   low capital costs because the size of the autoclave can be         reduced by:         -   a) diverting some of the concentrate to the roaster             (Fraction X); and         -   b)feeding the remaining concentrate (Fraction Y) to a small             autoclave at higher pulp density.

Embodiments of the present invention will be described with reference to the following examples which are presented for illustrative purposes only and are not intended to limit the scope of the invention.

EXAMPLE

One example of high temperature autoclaving of a chalcopyrite concentrate is shown in FIG. 2. When a chalcopyrite concentrate is processed at high temperature (>200° C.) in an oxidizing environment in an autoclave, the main reaction can be written as: ${{CuFeS}_{2} + {\frac{17}{4}O_{2}} + {H_{2}O}}->{{CuSO}_{4} + {\frac{1}{2}{Fe}_{2}O_{3}} + {H_{2}{SO}_{4}}}$

Under these conditions, copper is produced in the autoclave discharge as fully soluble copper sulphate, iron is precipitated in the residue as hematite Fe₂O₃, and two moles of SO₄ are produced per mole of chalcopyrite treated; that SO4 will have to be neutralized, consuming limestone.

Therefore, the chemical operating costs of such a process (HTPOX) are oxygen (for the oxidation inside the autoclave) and limestone (to neutralize the acid generated in the autoclave). In other words, these consumptions can be calculated for a chalcopyrite concentrate as 2.14 kg O₂/kgCu and 3.14 kg CaCO₃/kg Cu or, assuming a typical 25% Cu concentrate, as 618 kg O₂/T conc and 910 kg CaCO₃/T conc. While the costs of oxygen and limestone vary widely, in many cases their combined cost in the HTPOX process will be equivalent to 10% or more of the value of the copper recovered. There are no heating costs, since the oxidation reactions are generating sufficient heat that the process may be operated in a diluted or relatively low concentration manner to maintain the temperature. (i.e. for example 10% solids for a 25% S chalcopyrite concentrate).

If portions of the chalcopyrite concentrate are roasted prior to blending with unroasted materials following the present process, several events should happen at the same time:

-   -   1. The autoclave can now be operated at higher % solids, since         the % S of the blend is lower and there is less need to dilute         the autoclave feed to control the temperature.         -   For example, a blend at 15% sulphur may be operated at 20%             solids, which means that a plant would need an autoclave             volume less than half the size of the autoclave that would             have to process 100% unroasted feed. The capital savings             would compensate for the addition of a small roaster and             small acid plant to roast a portion of the feed. The roaster             would use air instead of oxygen to oxidize the sulphur; or             it could use oxygen-enriched air or oxygen, but it would be             low pressure oxygen instead of high pressure oxygen as used             during pressure oxidation.     -   2. Oxygen and limestone consumption would be significantly         reduced.         -   Because of the lower sulphur content of the blend feeding             the autoclave, oxygen consumption in the autoclave, and             limestone neutralization costs downstream of the autoclave,             may be significantly reduced.

Table 1 below shows the oxygen and limestone consumptions calculated for various blends, assuming all the sulphur is originating from the chalcopyrite. Various tests were conducted on a Canadian chalcopyrite concentrate, whereby various blends of unroasted concentrate and dead roasted concentrate were treated in an autoclave at 225° C. for 2 hours. The process intended is illustrated in FIG. 2. TABLE 1 Application to a 22.5% Cu, 32.3% S Canadian concentrate Consumptions % S in Acid generated % Calcine in Approx. O₂ CaCO₃ Autoclave % Cu in roaster Feed to autoclave % Solids kg/T cone kg/T cone feed Extracted kg/T cone 0 7.5 688 1009 32.3 98.0 0 20 8.5 595 809 25.9 98.8 192 30 10.0 484 709 22.7 97.8 288 40 12.0 415 609 19.5 99.9 384 50 15.0 347 509 16.3 95.3 480

Results indicate that after dead roasting a portion of the chalcopyrite concentrate, up to 50% of calcine could be blended with the unroasted concentrate. This blending did not affect copper extraction, but resulted in a 40-50% reduction of oxygen and limestone consumptions. The resulting blend still had sufficient sulphide sulphur for the autoclave operation to be auto-thermal (no heating costs), but also to operate at higher % solids. This will significantly reduce the capital expenditure (capex) of the autoclave circuit and subsequent solid/liquid separation circuit, to compensate for the additional cost of a small roaster and acid plant. Moreover, about 480 kg of acid is generated in the roaster.

-   -   3. Sales of pure acid         -   The portion of the blend going through the roaster will             generate commercial grade sulphuric acid. This acid could be             sold, or used up internally (i.e. heap leach operation),             generating additional revenues.     -   4. Recovery of gold         -   If gold is present in the chalcopyrite concentrate, it can             be easily recovered from the HTPOX residue (mainly hematite             Fe₂O₃), using cyanidation for example. In that case, cyanide             consumption as CNS will be low since the HTPOX conditions             oxidize the sulphide species to sulphate and very little             elemental sulphur remains in the final residue.         -   Alternatively, if PLATSOL® conditions are used during the             autoclaving, gold will be extracted together with copper and             can be recovered from the leach solution by various methods.

While the present invention has been described for what are presently considered the preferred embodiments, the invention is not so limited. To the contrary, the invention is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 

1. A hydrometallurgical process to leach at least one base metal from one or more base metal sulphide feeds comprising the steps of: providing a base metal sulphide feed; processing at least a portion of the feed by roasting to reduce the content of sulphur therein to a predetermined sulphur content level; directing some or all of the processed feed with or without some of the unprocessed feed to an oxidative leach step to carry out an oxidative leach step under conditions sufficient to recover at least a portion of the base metal from the feed, the predetermined sulphur content level being selected so as not to exceed that which is required to provide a source of heat for the oxidative leach step and to provide sufficient sulphate to complex the portion of the base metal being recovered.
 2. A process as defined in claim 1 wherein the directing step is carried out to recover substantially all of the recoverable base metals from the feed.
 3. A process as defined in claim 2 wherein the roasting step occurs under auto thermal conditions.
 4. A process as defined in claim 1 wherein the processing step includes the step of dividing the base metal sulphide feed into a first portion and a second portion, the first portion being subjected to the roasting step to form a roasted first portion.
 5. A process as defined in claim 4 wherein the second portion is directed to the oxidative leach step together with at least a portion of the roasted first portion.
 6. A process as defined in claim 1 wherein the oxidative leach step is carried out in one or more reaction vessels.
 7. A process as defined in claim 1, further comprising the step of recovering sulphur dioxide from the roasting step and converting the sulphur dioxide to sulphuric acid.
 8. A process as defined in claim 4 wherein the roasted first portion is at least partially depleted of sulphide.
 9. A process as defined in claim 5 wherein the roasted first portion is totally or partially blended with the second portion prior to delivery to the one or more reaction vessels.
 10. A process as defined in claim 5 wherein the step of directing the feed includes injecting the roasted first portion at different locations in the one or more reaction vessels.
 11. A process as defined in claim 6 wherein the one or more reaction vessels includes one or more autoclaves and/or one or more agitation tanks.
 12. A process as defined in claim 6 wherein one or more of the reaction vessels form one or more flow paths, which themselves may be in series or parallel relationship.
 13. A process as defined in claim 12 wherein one or more of the reaction vessels includes a number of compartments in series along one or more of the flow paths.
 14. A process as defined in claim 1 wherein the oxidative leach step is carried out in a heap.
 15. A process as defined in claim 4 further comprising the steps of: collecting a pressure leach slurry from the autoclave; and thereafter directing an additional portion of the roasted first portion to the pressure leach slurry, after it has been discharged from the autoclave.
 16. A process as defined in claim 1 wherein the feed includes gold, or platinum group metals and the oxidative leach step generates a slurry temperature within the autoclave ranging from about 190 to about 230 degrees Celsius.
 17. A process as defined in claim 16 wherein the gold, or platinum group metals are recovered using either a subsequent cyanidation step or a PLATSOL step.
 18. A process as defined in claim 1 wherein the oxidative leach step generates a slurry temperature within the reaction vessel ranging from about 40 to about 250 degrees Celsius.
 19. A process as defined in claim 1 wherein the pulp density in the feed to the oxidative leach step ranges from about 5 to about 30 percent.
 20. A process as defined in claim 4 wherein the first portion ranges from about 5 percent to about 100 percent of the feed.
 21. A process as defined in claim 1 wherein the one or more base metal sulphide feeds includes a concentrate formed from a flotation step, a gravity concentration step or a gravity/flotation concentration step.
 22. A process as defined in claim 21 wherein the first and second portions originate from different concentrates.
 23. A process as defined in claim 16 wherein the oxidative leach step occurs in the presence of sulphuric acid at a concentration not exceeding 80 g/L. 